The present invention relates generally to a system for measuring the level of a liquid in a tank, and more particularly, the present invention relates to an aircraft fuel gauging system which is capable of safely and reliably transmitting signals into and out of an aircraft fuel tank to probes located therein.
For safety and economic reasons, aircraft must have the capability of determining accurately the amount of fuel onboard. Thus, various fuel gauging systems and other sensing apparatus have been utilized to deliver information concerning the status of fuel carried by the aircraft to the pilot. In general, all these systems require portions of electronic circuits, such as, electrical wiring, probes, sensors, etc., to extend inside the fuel tank.
Examples of known aircraft fuel gauging systems are disclosed in U.S. Pat. No.: 5,602,333 issued to Larrabee et al.; U.S. Pat. No. 4,918,619 issued to Orloff et al.; U.S. Pat. No. 4,908,783 issued to Maier; U.S. Pat. No. 4,872,120 issued to Orloff et al.; U.S. Pat. No. 4,731,730 issued to Hedrick et al.; U.S. Pat. No. 4,451,894 issued to Dougherty et al.; U.S. Pat. No. 4,487,066 issued to Pardi et al.; U.S. Pat. No. 4,173,893 issued to Hedrick; and U.S. Pat. No. 4,090,408 issued to Hedrick.
As a result of a recent airplane accident, namely the xe2x80x9cTWA 800xe2x80x9d accident, the FAA and the aviation industry have focused a great deal of attention on the electrical systems in airplane fuel tanks. Such electrical systems, such as pumps and fuel gauging, have been found with damaged wiring insulation which invites the possibilities of electrical shorts in the fuel tank. Such conditions violate the original design intent of providing electrical circuit isolation in the fuel tank.
The FAA has responded to these concerns by issuing several Airworthiness Directives requiring the modification of airplanes to eliminate these potential hazards. Additional requirements are expected to be issued that will further restrict electrical power in aircraft fuel tanks. These new requirements impose additional costs on operators of aircraft, both for initial aircraft modifications and repair and for continued maintenance of these fuel tank electrical systems. Many of the solutions to these problems involve the addition of wire shielding which adds weight to the airplane; however, the additional weight results in increases in the cost to fly the airplane.
As referenced above, U.S. Pat. No. 5,602,333, which issued to Larrabee et al. and which is assigned to the assignee of the present application, discloses a scheme of multiplexing several fuel capacitance probes on a single set of wires. A schematic representation of the system is illustrated in FIG. 2. The probe capacitance and a fixed inductor form a resonant circuit having a resonant frequency which is a function of the probe capacitance which is in turn a function of the fuel height at the probe. While this system offers improvements over previous technology and limits the electrical wiring in the tank, it still relies on the use of electrical wiring to bring signals in and out of the fuel tank.
Therefore, although various fuel gauging systems are known in the art which may be satisfactory for their intended purposes, there is a need for a novel fuel gauging system which requires no electrical wires to enter, or exit, the fuel tank. In addition, the system should permit multiplexing of a large number of fuel probes and should weigh less then currently available systems.
With the foregoing in mind, a primary object of the present invention is to completely eliminate the use of electrical wires to transmit signals in and out of fuel tanks.
Another object of the present invention is to provide an accurate and lightweight aircraft fuel gauging system which includes the multiplexing of a relatively large number of fuel probes.
More specifically, the present invention provides an apparatus having at least one probe for measuring the level of a liquid in a tank. The probe is located in the tank for at least partial immersion in the liquid and is capable of receiving an input signal and providing an output signal in response to the input signal. The output signal is utilized to determine the level and amount of liquid in the tank.
The improvement provided by the apparatus of the present invention is the use of a fiber optic element for transmitting the input signal to the probe. The input signal is generated by a light source externally of the tank and is transmitted into the tank by the fiber optic element.
According to one embodiment of the present invention, the same fiber optic element transmits both the input signal from the light source and the output signal from the probe. The output signal is transmitted to a light detector located exteriorly of the tank.
According to an alternate embodiment of the present invention, a second separate fiber optic element is used to transmit a measurement light signal to the probe and to transmit the output signal from the probe to a light detector located exteriorly of the tank.